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September 2011

September 14, 2011

In my last post "Going Faster Has a Price" I discussed the issues with transmitting bits represented by two states at faster data rates and the problems of inherent loss in the media, ISI and many other phenomenon that screw up the signal. Through careful channel design and active means, engineers can transmit and recover bits over copper cable and back planes with ever greater rates. For example, National Semiconductor and Molex demonstrated 25Gbps+ communications over a back plane at DesignCon 2011 this year. But how long can the industry keep doing this without changing the way we define a bit on a backplane?

This problem is not a new one... as a matter of fact, it is a very old one going back to the early telecom days of modems. In the early days of circuit switched (voice) networks, filters were placed in the system to limit the bandwidth of the signal to around 3KHz which was enough to reconstruct a human female voice without distortion. This was done primarily as a means to frequency multiplex multiple telephone circuits on a single microwave transmission between towers (before fiber-optic lines). So when people tried to move "bits", they were limited to the 3Khz bandwidth.Enter the Shannon-Hartley Capacity theorem (see below).

What this says is the maximum capacity of a channel to carry information is a function of the bandwidth (B) in Hertz and the Signal to Noise Ratio (S/N) which has no units. So as your noise goes up, your capacity to move information goes down. This plagued early engineers and limited the amount of information that could be moved through the network. Early modems used Frequency Shift Keying (FSK). One frequency was used to indicate a "0" state and another to represent a "1" state. The frequencies where chosen so that they would pass through the 3Khz limit of the channel and could be filtered from the noise. The problem is that you couldn’t switch between them faster than the bandwidth of the channel so you were still limited to the 3KHz... so how did they get around this? They used Symbol Coding.

Symbol coding basically combines groups of bits into a single symbol. That symbol can be represented by a frequency carrier and a combination of amplitude and phase. This led to the development of Quadrature Phase Shift Keying (QPSK) or Quadrature Amplitude Modulation (QAM) techniques which are in use today in modern cable modems. The group of bits can be sent all at once instead of one bit at a time... clever! However, it comes at a cost and a fair amount of complexity relegated to the world of digital signal processing.

But what about the high-speed digital signal path between two systems in our modern Internet? Today they use scrambled Non-Return-to-Zero (NRZ) coding which prevents DC wander and EMI issues... but it is still either a "0" or a "1" state - two levels representing the state of a bit. Will this medium ever move to other coding schemes to get more data through the channel as the early telephone system did? It might. Intel and Broadcom are both pushing for a standard that uses multiple levels and symbol encoding for 25 Gbps and beyond. This has the added benefit that more bits can be sent in a single transmission of a symbol. This is already being done today in Ethernet for the 10/100/1000 CAT-5/6/7 standards over UTP cable where the bandwidth of the channel is limited to around 350 Mhz. Will we see this at 25 Gbps and beyond? Possibly...

The problem with this method is power. It takes DSP technology at each end of the channel to code and recover the signals adding energy consumption to the mix. With thousands of channels in a modern data center, that power can add up really fast. NRZ techniques are very low in power consumption. National Semiconductor has produced devices that can move data at rates of 28 Gbps over copper media and back-planes at very low power consumption - something multi-level systems will find difficult to do. The industry agrees and is pushing back on the multi-level proposals.

There may come a day beyond 28 Gbps where there is no alternative but to go to multi-level symbol encoded systems, but I think that may be some time off in our future when 100 Gbps is far more common - perhaps even to your cell phone! Till next time...